The present invention relates to a fuel tank inerting system and, more particularly, a dispatch critical fuel tank inerting system that would allow for a reduction in the inspection burden imposed by the flammability reduction regulations and possibly allow for a lighter design for some of the electrical and mechanical components that reside in the fuel tanks of commercial aircraft.
Fuel tanks of an aircraft represent vulnerable areas on all aircraft for the potential for flame initiation due to the existence of fuel vapor and oxygen concentration levels therein. Fuel tank inerting systems are currently used to reduce oxygen concentration levels within fuel tanks of some military aircraft in order to significantly reduce the vulnerability of military aircraft to hostile munitions.
As is known in the art, as oxygen concentration levels within a fuel tank increase, likelihood of flame initiation and propagation and likelihood of a possible explosion also increases. This threat exists since fuel vapors generally mix with ambient air that has a 21% concentration of oxygen.
Some aircraft, principally military aircraft, are equipped with fuel tank inerting systems, which supply nitrogen gas to purge fuel tanks and effectively reduce oxygen concentration levels therein. Of available types of fuel tank inerting systems, the most desirable from a weight, capacity, and ground service requirements stand point is an inert gas generation system that utilizes pressurized air supplied by engine bleed from a gas turbine engine or other airborne source of pressurized air. This air is separated into an oxygen rich component, which is exhausted overboard, and an oxygen depleted or inert gas component, which is fed to the fuel tank.
In commercial aircraft ignition applications, other requirements, such as reliability, maintenance and cost of internal components and systems can be more stringent. Military inerting systems often utilize complicated and less reliable components to provide nitrogen-enriched air to each fuel tank, for all perceivable mission conditions. During combat military missions, threats from hostile munitions are highly probable. The military type systems have a poor reliability history, with high maintenance costs and are oversized for the vast majority of typical commercial aircraft operations.
The primary source of flammability exposure for current commercial aircraft is in the center fuel tanks, particularly if located adjacent to heat sources. Thus, a primary desire exists in commercial aircraft applications to reduce flammability exposure in center fuel tanks, to a level that is similar to that of the wing fuel tanks. Additionally, reducing exposure in wing fuel tanks can also be desirable when aircraft design characteristics result in high flammability exposure or when additional reduction in wing fuel tank flammability exposure is desired.
Additionally, it is also desirable for the fuel tank to be inert during both ground and flight conditions. Further, variations in oxygen concentrations throughout tanks must be minimized to achieve a uniform level of inert content, without over sizing the inerting system.
Current inerting systems provide fuel tank safety enhancement and are typically not dispatch critical. Referring to FIG. 1, the fuel tank inerting system 100 comprises a single thermal control unit 102 feeding a single bank 104 of three air separation modules 106. The thermal control unit 102 controls the temperature of an inlet flow 108 to the air separation module 106 to a fixed temperature and regulates the pressure in climb and cruise to a maximum of 45±6 psig. During most of the cruise segment, the bleed pressure is well below the regulation set point so the system 100 operates with the regulator 110 fully open. During the descent phase, the system 100 obtains a boost in the purity of the nitrogen enriched air provided to the fuel tank by compressing the inlet flow. At the top of descent, a turbo compressor shut-off valve 112 is opened and bleed air is supplied to a drive turbine 114 of a turbo compressor 118 so that the compressor 116 boosts the pressure available from the aircraft bleed system.
This conventional system 100 is sized to provide the flow and level of nitrogen enrichment that is high enough during all flight segments so that the fleet flammability exposure of the center wing tank is less than 3% of the operational time. To ensure that the center wing tank is protected from possible latent ignition sources during the flights conducted with the system inoperable (INOP), the operator is required to conduct regular inspections of the pumps and wiring within the tank.
The components that drive the INOP rate of the system 100 are the system shut-off and temperature control valves and, to a lesser extent, the turbo compressor and its shut-off valve. The nitrogen separation performance of the air separation module is monitored on every flight using the oxygen sensor and will be maintained on a hard-timed removal schedule; it is not expected to be a significant contributor to dispatch failures.
As can be seen, there is a need for a fuel tank inerting system that may meet the challenges imposed by the redundancy and reliability needed for a dispatch critical system that complies with commercial transport operational interrupt requirements.